1. Field of the Invention
This invention relates generally to sinusoidal oscillators, and, more particularly, the invention disclosed relates to an improved control circuit for maintaining constant amplitude of the sinusoidal output signal of an oscillator circuit with minimum input power requirements.
2. Description of the Prior Art
Variable reluctance pressure transducers and other electrical circuits often require highly stable oscillators with constant amplitude output signals. In the transducer field it has been found that excitation by a sinusoidal input signal provides an advantage in reducing radio frequency interference and ease of amplification due to lower frequency band pass requirements.
Prior art oscillators have achieved amplitude stabilization through various methods. A first method involved the use of a variable impedance connected in the positive feedback path to attenuate the amplitude of the AC signal being fed back. Control of the variable impedance was achieved through a DC control voltage derived from the AC output signal. This control technique was highly sensitive to changes in input voltages or temperature variation and efficiency of variable impedance controlled oscillators was low.
Alternate control techniques made use of an AC to DC convertor for deriving a DC signal from the AC output signal. The DC signal was then provided through a negative feedback path including a differential amplifier to control the DC current supplied to the input of the oscillator. The differential amplifier compares the DC converted signal to a DC reference signal for control of the sine wave amplitude. This prior art technique as exemplified in U.S. Pat. No. 3,763,444 to Tavis proved highly insensitive to temperature variations and input voltage changes due to the use of the differential amplifier in the negative feedback path of the oscillator.